238 U Neutron Capture with the Total Absorption Calorimeter

n_TOF Annual Meeting, Lisbon, Dec 2011

238U Neutron Capture with the Total Absorption Calorimeter

Toby WrightUniversity Of Manchester


238U (n,γ) TAC measurement

• Featured on the NEA high priority list• High accuracy (n,γ) 238U cross-section

measurement up to 100 keV• Joint future analysis with the same

measurement using C6D6 detectors at nTOF and GEEL

• Aim of reducing the uncertainty in the cross-section from ~5% to ~2%


JEFF 3.1.1 238U (n,γ) cross-section

TACC6D6


The 238U Sample

• 6.125 ± 0.002 Grams• Isotopic analysis done in 1984

• <1 ppm 233U• <1ppm 234U• ~11ppm 235U• <1ppm 236U

• Wide sample, with perfect alignment it covers ~97% of the beam (53.90 x 30.30 mm)

• Encased with ~60 microns of Al, ~75 microns Kapton

The 238U arrived as a powder, suspended in solution


Other Samples

• Carbon and Gold samples were measured to determine the background due to scattered neutrons, and normalisation respectively

• Measurements were done with no sample at all present, and also with the 238U sample but no beam to determine the backgrounds present in the measurement

The 3 samples measured. From L to R: 238U, Gold, Carbon


Experimental Setup

• Data is taken at a 250MHz sampling rate, for 32ms allowing measurements to be taken from 0.3eV

• The raw data is stored in CERN’s CASTOR facility after zero data suppression


The Measurement

Sample Protons Time238U 4.12e+17 24d 16:15:02Nothing (sample out) 1.05e+17 6d 16:56:14

Gold 6.74e+16 6d 10:30:54

Carbon 7.05e+16 2d 17:49:28

• Lasted around 41 days in total• Used 4 different proton intensities (0.5, 0.9, 3 and 8e12ppp) to

investigate the effect of pile up due to the large mass and therefore large counting rate from the sample

• A spherical C12H20O4(6Li2) surrounded the samples to reduce the neutron induced background

• The whole running went very smoothly• Due to extremely little sample encapsulation, it should be

possible to measure up to higher neutron energies than ever before with the TAC (up to ~100keV)


TAC Calibrations - Energy• Three calibration sources used:

– 137Cs (661.7 keV)– 88Y ( 0.898 and 1.836 MeV)– AmBe (4.44 MeV)


88Y


88Y – Fit Background

• Background fitting and peak finding done with TSpectrum class in ROOT


88Y – Fit Gaussian


Detector Drift

• Very little drift over time

• Only need to use one set of calibration values for the whole measurement

Detector Number

Chan

nel N

umbe

r


88Y Fits


AmBe Fits


Dst creation investigation – 60Co

Change the value of ‘slowtau’ in the routine. This is related to the decay time of the slow component of the signal


Dst creation investigation - AmBe


Dst creation investigation – Detector Resolution


88Y Fits (version 10)


AmBe Fits (version 10)


TAC AmBe Plot

Peak positionGaussian peak position

• Gaussian fits are not accurate due to the presence of the single escape peak

• More accurate calibration when the peak position, as found by the Tspectrum peakfinder is taken

4.44 MeV


Dst creation Investigation


Proton Intensity


Counting Rate


Preliminary Results


Preliminary Results


First ResonanceThird Resonance

Second Resonance

First Resonances


Statistics

Shown here is resonances around 1.65keV. This is with approximately 15% of the total statistics acquired during the 238U runs.


Effect of the gamma flash on the TAC


Effect of the gamma flash on the TAC


Conclusions• The experiment was

successfully completed• A combination of n_TOF’s high

instantaneous neutron flux, the massive sample and the TAC’s efficiency should make this the most accurate (n,γ) 238U cross-section measurement ever done

• The preliminary data analysis done looks very promising

Documents

238 U Neutron Capture with the Total Absorption Calorimeter